Leasy Zhang/shiwangzhihe@gmail.com

java-collections-conversions-2/src/test/java/com/baeldung/convertlisttomap/ListToMapUnitTest.java

@@ -0,0 +1,102 @@
+package com.baeldung.convertlisttomap;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.function.BiConsumer;
+import java.util.function.Supplier;
+import java.util.stream.Collectors;
+
+import org.junit.Test;
+
+public class ListToMapUnitTest {
+
+    @Test
+    public void givenAList_whenConvertWithJava8GroupBy_thenReturnMap() {
+        List<String> strings = Arrays.asList("List", "Map", "Set", "Tree");
+
+        Map<Integer, List<String>> convertedMap = new HashMap<>();
+
+        Supplier<List<String>> listSupplier = () -> {
+            return new ArrayList<>();
+        };
+
+        Supplier<Map<Integer, List<String>>> mapFactory = () -> {
+            return new HashMap<>();
+        };
+        convertedMap = strings.stream()
+            .collect(Collectors.groupingBy(String::length, mapFactory, Collectors.toCollection(listSupplier)));
+
+        assertEquals(2, convertedMap.size());
+        assertTrue(convertedMap.get(3)
+            .contains("Map"));
+    }
+
+    @Test
+    public void givenAList_whenConvertWithJava8Collect_thenReturnMap() {
+        List<String> strings = Arrays.asList("List", "Map", "Set", "Tree");
+
+        Map<Integer, List<String>> convertedMap = new HashMap<>();
+
+        Supplier<Map<Integer, List<String>>> mapFactory = () -> {
+            return new HashMap<>();
+        };
+
+        Supplier<List<String>> listSupplier = () -> {
+            return new ArrayList<>();
+        };
+
+        BiConsumer<Map<Integer, List<String>>, String> accumulator = (response, element) -> {
+            Integer key = element.length();
+            List<String> values = response.getOrDefault(key, listSupplier.get());
+            values.add(element);
+            response.put(key, values);
+        };
+
+        BiConsumer<Map<Integer, List<String>>, Map<Integer, List<String>>> combiner = (res1, res2) -> {
+            res1.putAll(res2);
+        };
+
+        convertedMap = strings.stream()
+            .collect(mapFactory, accumulator, combiner);
+
+        assertEquals(2, convertedMap.size());
+        assertTrue(convertedMap.get(3)
+            .contains("Map"));
+    }
+
+    @Test
+    public void givenAList_whenConvertWithCollectorToMap_thenReturnMap() {
+        List<String> strings = Arrays.asList("List", "Map", "Set", "Tree");
+
+        Map<Integer, List<String>> convertedMap = new HashMap<>();
+
+        Supplier<Map<Integer, List<String>>> mapFactory = () -> {
+            return new HashMap<>();
+        };
+
+        Supplier<List<String>> listSupplier = () -> {
+            return new ArrayList<>();
+        };
+
+        convertedMap = strings.stream()
+            .collect(Collectors.toMap(String::length, (p) -> {
+                List<String> strs = listSupplier.get();
+                strs.add(p);
+                return strs;
+            }, (existing, replacement) -> {
+                existing.addAll(replacement);
+                return existing;
+            }, mapFactory));
+
+        assertEquals(2, convertedMap.size());
+        assertTrue(convertedMap.get(3)
+            .contains("Map"));
+    }
+
+}